Round lens doesn’t capture the photo. Lens only passes the image inside the camera. Within the camera body image sensor (in the old cameras film strip) captures the images. Since the shape of the image sensor (or film strip) is in rectangular shape we get rectangular photos. This rectangle covers only about 56% of the actual circular image that passes through the lens. So, the remaining 44% image is getting lost.

Having seen HOW part, now let us move to WHY part.

Why we are discarding 44% of the image that is coming into the camera through the lens? Answer is – for convenience sake.

Traditionally, in film cameras, there used to be rectangular frames connected to each other on a long film strip. If circular photo need to be captured on the film, we need to have film strip that is a series of circular frames connected to each other – it is very difficult to handle such a film. Another option is to capture the circular image on a rectangular frame where diameter of the image is equal to height of the frame. In this case, four corners of the file will be wasted, objects in the image will look relatively smaller (compared to what we are seeing now).

Even if we are ready to waste some portion of the film and get smaller images on the film (as mentioned above), it is again difficult to print the photos in circular shape – need to waste the corners of the photo print paper or need to zoom the circular negative and print in rectangular shape. So it has been decided to loose 44% of the circular image and capture only 56% in the rectangular shape.

Whatever has been done in film days is being continued in digital era also. Since all the display devices we are using today – computers, TVs, tabs, mobiles, etc. are in rectangular shape, the same 56% image capture mechanism is being followed today also.

Also images produced by many lenses tend to get distorted at the edges of the images. By cropping most of the rounded edges and taking only the rectangular portion we are reducing that distortion in the final photos.

Having seen HOW and WHY, you may be left with one more question – why not make the lenses also in rectangular shape?

The mechanical process involved in grinding and polishing lens favors making lenses in round shape over rectangular shape, hence circular lenses are cheaper. [you might be aware that event the lenses used in spectacles are actually made in circular shape and later cut into rectangular shape based on the size required for the spectacle wearer!]

Even if the camera lenses are made in rectangular shape, to avoid the distortion of the image age the edges, image need to be chopped off on alfour sides before capturing the photo on the image sensor.